Exhaust gases from diesel engines contain particulate matter (PM) based on carbonaceous soot and SOFs (soluble organic fractions) of high-boiling-point hydrocarbons, and the release of PM into the atmosphere is likely to exert adverse effects on humans and environment. Exhaust pipes connected to diesel engines are thus conventionally provided with ceramic honeycomb filters (simply called “honeycomb filters” below) for capturing PM.
FIGS. 1(a) and 1(b) show one example of honeycomb filters for capturing PM to clean the exhaust gas. A honeycomb filter 10 comprises a ceramic honeycomb structure having porous cell walls 2 for constituting large numbers of outlet-side-sealed flow paths 3 and inlet-side-sealed flow paths 4 and a peripheral wall 1, and upstream-side plugs 6a and downstream-side plugs 6c for sealing the exhaust-gas-inlet-side end surface 8 and exhaust-gas-outlet-side end surface 9 of the outlet-side-sealed flow paths 3 and inlet-side-sealed flow paths 4 alternately in a checkerboard pattern.
As shown in FIG. 2, this ceramic honeycomb filter 10 is received in a metal container 12, axially gripped by a support member 14, and sandwiched by support members 13a, 13b. The support member 14 is generally formed by a metal mesh and/or ceramic material. When the ceramic honeycomb filter 10 attached to a diesel engine is used, mechanical vibration and shock are conveyed from the engine, the road, etc. to the ceramic honeycomb filter 10 via the support members 13a, 13b and 14, so that the ceramic honeycomb filter 10 is subjected to a load. Particularly because industrial ceramic filters used for construction machines, etc., or large ceramic honeycomb filters of more than 200 mm in outer diameter are subjected to a larger load by vibration and shock, they are required to have higher strength than that of conventional ones.
The ceramic honeycomb filters should have three important characteristics, particulate-matter-capturing efficiency, pressure loss, and particulate-matter-capturing time (time period from the start of capturing particulate matter, during which pressure loss reaches a predetermined level). Particularly the capturing efficiency and the pressure loss are in a reciprocal relation; higher capturing efficiency results in larger pressure loss and shorter capturing time, and lower pressure loss results in longer capturing time and poorer capturing efficiency. To meet all of these contradictory filter characteristics, investigation has conventionally been conducted to develop technologies for controlling the porosity and average pore size of the ceramic honeycomb structures, and pore sizes on their cell wall surfaces.
JP 61-129015 A discloses a low-pressure-loss, exhaust-gas-cleaning filter having pores on the cell wall surfaces, the pores comprising small pores having pore sizes of 5-40 μm and large pores having pore sizes of 40-100 μm, the number of said small pores being 5-40 times that of said large pores, thereby having high capturing efficiency from the start. This reference describes that pores in the cell walls preferably have an average pore size of more than 15 μm and a cumulative pore volume of 0.3-0.7 cm3/g. Because the porosity P (% by volume) of cell walls can be calculated from the true specific gravity ρ (=2.5 g/cm3) and cumulative pore volume V (cm3/g) of a cordierite material by the formula of P=100×V×ρ/(1+V×ρ), the cumulative pore volume of 0.3-0.7 cm3/g in said cell walls can be converted to the porosity of 42.8-63.6% by volume. The pore size distribution line shown in FIG. 4 in JP 61-129015 A indicates that the honeycomb filters of Examples 1, 2, 5 and 6 have cumulative pore volumes of 0.58 cm3/g (porosity 59%), 0.4 cm3/g (porosity 50%), 0.7 cm3/g (porosity 64%) and 0.3 cm3/g (porosity 43%), respectively, and average pore sizes of 40 μm, 35 μm, 44 μm and 15 μm, respectively.
However, particularly when used for industrial ceramic filters for construction machines, etc. or large ceramic honeycomb filters of more than 200 mm in outer diameter, the honeycomb filters of Examples 1, 2 and 5 have too large average pore sizes or porosities, resulting in insufficient strength, and the honeycomb filter of Example 6 has too small porosity, resulting in high pressure loss. Namely, the honeycomb filters of Examples 1, 2, 5 and 6 do not have both low pressure loss and high strength.
JP 2002-219319 A discloses a porous honeycomb filter, in which the volume of pores having pore sizes of less than 10 μm is 15% or less of the total pore volume, the volume of pores having pore sizes of 10-50 μm is 75% or more of the total pore volume, and the volume of pores having pore sizes exceeding 50 μm is 10% or less of the total pore volume. This reference describes that this porous honeycomb filter has high particulate-matter-capturing efficiency while preventing increase in pressure loss due to the clogging of pores. However, the porous honeycomb filter described in JP 2002-219319 A does not have sufficiently low pressure loss particularly when used as industrial filters for construction machines, etc., or large filters of more than 200 mm in outer diameter.
JP 2004-322082 A discloses a ceramic honeycomb filter having a total pore volume of 0.55-0.80 cm3/g (corresponding to porosity of 59-67% when converted by the above formula), the volume of pores of 100 μm or more being 0.02-0.10 cm3/g. This reference describes that this filter has low pressure loss and high strength. In high-porosity ceramic honeycomb filters as described in JP 2004-322082 A, however, there is still room for improvement to meet both low pressure loss and sufficient strength to withstand mechanical vibration and shock, particularly when used as industrial filters for construction machines, etc. or large filters of more than 200 mm in outer diameter.
JP 2004-250324 A discloses a method for producing a ceramic honeycomb structure using a cordierite-forming material containing 10-20% by mass of silica particles, in which the percentage of particles having particle sizes of 75-250 μm is more than 1% by mass and 10% by mass or less. This reference describes that the ceramic honeycomb filter has low pressure loss and high strength. However, the use of quartz and silica particles shown in Table 2 of JP 2004-250324 A provides ceramic honeycomb filters with too high percentage of fine pores, resulting in high pressure loss. In addition, because the ceramic honeycomb filters described in JP 2004-250324 A have high porosity, there is still room for improvement to meet both low pressure loss and sufficient strength to withstand mechanical vibration and shock, particularly when used as industrial filters for construction machines, etc. or large filters of more than 200 mm in outer diameter.
JP 2003-193820 A discloses a ceramic honeycomb filter having cell walls having porosity of 60% or more and an average pore size of 15 μm or more, the maximum inclination of a cumulative pore volume distribution curve plotted with the pore size being 0.7 or more. This reference describes that the ceramic honeycomb filter has low pressure loss, and high strength and durability. However, because the ceramic honeycomb filters described in JP 2003-193820 A have high porosity, there is still room for improvement to meet both low pressure loss and sufficient strength to withstand mechanical vibration and shock, particularly when used as industrial filters for construction machines, etc. or large filters of more than 200 mm in outer diameter.